1. Field of the Invention
This invention relates to a hydrostatic power unit in the form of a variable displacement machine with a continuously variable displacement volume and which can be operated as a pump and as a motor. The power unit is in a drive connection with an internal combustion engine. When operated as a pump, a suction side of the power unit sucks hydraulic fluid from a tank and delivers the hydraulic fluid into a delivery line that leads to at least one consumer. When operated as a motor, the power unit is a hydraulic starter to start the internal combustion engine, with hydraulic fluid from a hydraulic accumulator supplied to the suction side of the power unit.
2. Description of Related Art
Hydrostatic power units are known which are driven by the running internal combustion engine and which, when operated as a pump, are used to supply at least one consumer with hydraulic fluid. When operated as a motor, the power units are used as a hydraulic starter to start the shut-off internal combustion engine. These power units are used in mobile machines, such as industrial trucks, agricultural equipment, forestry equipment, and construction equipment, such as excavators, wheeled and telescoping loaders, tractors, combine harvesters, forage harvesters, and sugar beet or potato diggers, for example.
To reduce fuel consumption during pauses or interruptions in work, a start-stop function is provided for the internal combustion engine, in which the internal combustion engine, when running at no load, is shut off during pauses or interruptions in the work, and is automatically restarted when torque is requested by a work function or by the traction drive. A start-stop function of the internal combustion engine of this type can be reliably and robustly accomplished with a hydraulic starter.
EP 2 308 795 A1 describes a hydrostatic power unit which, when operated as a pump in which the power unit is driven by an internal combustion engine, sucks hydraulic fluid via a suction side out of a tank and supplies the consumer of the working hydraulics with hydraulic fluid. When operated as a motor, in which hydraulic fluid is delivered to the suction side from a hydraulic accumulator and the power unit is driven by the hydraulic fluid from the hydraulic accumulator, the power unit is used as a hydraulic starter to start the shut-off internal combustion engine.
If the power unit is a variable displacement machine with a continuously variable displacement volume, for operation as a motor the power unit must be set to a position with the maximum displacement volume to provide the output of sufficient torque required to start the internal combustion engine. From EP 2 308 795 A1 (FIG. 2), it is known that a variable displacement device which controls the displacement volume of the power unit can be connected with the hydraulic accumulator so that when the internal combustion engine is shut off, the displacement volume control device of the power unit can be actuated with the hydraulic fluid from the hydraulic accumulator. For this purpose, the hydraulic accumulator is in communication via a pressure reducer valve with a charging pressure circuit which, when the internal combustion engine is running, supplies the variable displacement device of the power unit with hydraulic fluid. A supply of this type of the variable displacement device with hydraulic fluid from the hydraulic accumulator, however, requires a great deal of extra construction effort and expense. The charge pressure circuit is connected to the hydraulic accumulator downstream of a valve device which controls the operation of the power unit as a motor by a connection between the hydraulic accumulator and the suction side of the power unit, so that the displacement volume control device is actuated with the hydraulic fluid from the hydraulic accumulator into the position with the maximum displacement volume chronologically immediately prior to the starting process of the internal combustion engine. However, on account of the setting of the displacement volume control device at the beginning of the starting process, there is a time lag in the starting process of the internal combustion engine. This has disadvantages for a start-stop function in which the starting process of the internal combustion engine must occur in a short period of time.
Therefore, it is an object of the invention to provide a hydrostatic power unit of the general type described above but which with little added construction effort or expense makes it possible to start the internal combustion engine in a shorter period of time with the power unit operating as a motor.